Osmoregulation is the control of the water content of body fluids. It is part of
homeostasis, the maintenance of a constant internal environment.
It is important that cells are surrounded by tissue fluid of a similar water potential to their
own contents, to avoid too much water loss or gain which could disrupt metabolism. You
have seen that water is lost from the fluid inside a nephron as it flows through the
collecting duct. The permeability of the walls of the distal convoluted tubule and
collecting duct can be varied.
If they are permeable, then much water can move out of the tubule and the urine
becomes concentrated. The water is taken back into the blood and retained in the body.
If they are made impermeable, little water can move out of the tubule and the
urine remains dilute. A lot of water is removed from the body.
ADH
ADH is antidiuretic hormone. It is secreted from the anterior pituitary gland into the
blood.
When the water potential of the blood is too low (that is, it has too little water in it), this is
sensed by osmoreceptor cells in the hypothalamus. The osmoreceptor cells are
neurones (nerve cells). They produce ADH, which moves along their axons and into the
anterior pituitary gland from where it is secreted into the blood.
The ADH travels in solution in the blood plasma. When it reaches the walls of the
collecting duct, it makes them permeable to water. Water is therefore reabsorbed from
the fluid in the collecting duct and small volumes of concentrated urine are produced.
When the water potential of the blood is too high (that is, it has too much water in it), this
is sensed by the osmoreceptor cells and less ADH is secreted. The collecting duct walls
therefore become less permeable to water and less is reabsorbed into the blood. Large
volumes of dilute urine are produced.
Negative feedback
The mechanism for controlling the water content of the body, using ADH, is an example
of negative feedback.
When the water potential of the blood rises too high or falls too low, this is sensed by
receptor cells. They cause an action to be taken by effectors which cause the water
potential to be moved back towards the correct value.
In this case, the receptors are the osmoreceptor cells in the hypothalamus, and the
effectors are their endings in the anterior pituitary gland that secrete ADH.
